Cancer Stem Cells (CSCs).
In recent years, a new model of tumorigenesis has gained wide acceptance, where it is hypothesized that only a small fraction of the entire tumor mass are responsible for the tumorigenic activities within the tumor, whereas the old or clonal genetic model posits that all the mutated tumor cells contribute equally to such tumorigenic activities. This small fraction of tumorigenic cells, according to the new model, are transformed cells with stem-cell-like qualities and are called “cancer stem cells” (CSCs). Bonnet and Dick first demonstrated, in vivo, the presence of CSCs in acute myeloid leukemia (AML) during the 1990s. Their data showed that only a small subpopulation of human AML cells had the ability to transfer AML when transplanted into immunodeficient mice while other AML cells were incapable of inducing leukemia. Later, these CSCs were shown to have the same cellular markers, CD34+/CD38−, as primitive hematopoietic stem cells [1]. Since then, researchers have found CSCs conclusively in various types of tumors including those of the brain, breast, skin, prostate, and so on.
The CSC model of tumorigenesis would explain why tens or hundreds of thousands of tumor cells need to be injected into an experimental animal in order to establish a tumor transplant. In human AML, the frequency of these cells is less than 1 in 10,000 [2]. Even though rare within a given tumor cell population, there is mounting evidence that such cells exist in almost all tumor types. However, as cancer cell lines are selected from a subpopulation of cancer cells that are specifically adapted to grow in tissue culture, the biological and functional properties of cancer cell lines can undergo dramatic changes. Therefore, not all cancer cell lines contain CSCs.
Cancer stem cells share many similar traits with normal stem cells. For example, CSCs have self-renewal capacity, namely, the ability to give rise to additional tumorigenic cancer stem cells, typically at a slower rate than other dividing tumor cells, as opposed to a limited number of divisions. CSCs also have the ability to differentiate into multiple cell types, which would explain histological evidence that not only many tumors contain multiple cell types native to the host organ, but also that heterogeneity is commonly retained in tumor metastases. CSCs have been demonstrated to be fundamentally responsible for tumorigenesis, cancer metastasis, and cancer reoccurrence. CSCs are also called tumor initiating cells, cancer stem-like cells, stem-like cancer cells, highly tumorigenic cells, tumor stem cells, solid tumor stem cells, or super malignant cells.
The existence of cancer stem cells has fundamental implications for future cancer treatments and therapies. These implications are manifested in disease identification, selective drug targeting, prevention of cancer metastasis and recurrence, and development of new strategies in fighting cancer.
The efficacy of current cancer treatments are, in the initial stages of testing, often measured by the size of the tumor shrinkage, i.e., the amount of tumor mass that is killed off. As CSCs would form a very small proportion of the tumor and have markedly different biologic characteristics than their more differentiated progenies, the measurement of tumor mass may not necessarily select for drugs that act specifically on the stem cells. In fact, cancer stem cells appear to be resistant to radiotherapy (XRT) and also refractory to chemotherapeutic and targeted drugs [3-5]. Normal somatic stem cells are naturally resistant to chemotherapeutic agents—they have various pumps (such as MDR) that pump out drugs, and DNA repair proteins. Further, they also have a slow rate of cell turnover while chemotherapeutic agents target rapidly replicating cells. Cancer stem cells, being the mutated counterparts of normal stem cells, may also have similar mechanisms that allow them to survive drug therapies and radiation treatment. In other words, conventional chemotherapies and radiotherapies kill differentiated or differentiating cells, which form the bulk of the tumor that are unable to generate new highly tumorigenic cancer stem cells. The population of cancer stem cells that gave rise to the differentiated and differentiating cells, on the other hand, could remain untouched and cause a relapse of the disease. A further danger for conventional anti-cancer therapy is the possibility that chemotherapeutic treatment leaves only chemotherapy-resistant cancer stem cells, and the ensuing recurrent tumor will likely also be resistant to chemotherapy.
Since the surviving cancer stem cells can repopulate the tumor and cause relapse, it is imperative that anti-cancer therapies include strategies against CSCs (see FIG. 1). This is akin to eliminating the roots in order to prevent dandelions from regrowth even if the weed's ground level mass has been cut [6]. By selectively targeting cancer stem cells, it becomes possible to treat patients with aggressive, non-resectable tumors and refractory or recurrent cancers, as well as preventing the tumor metastasis and recurrence. Development of specific therapies targeting cancer stem cells may improve survival and the quality of life of cancer patients, especially for sufferers of metastatic cancers. The key to unlocking this untapped potential is the identification and validation of pathways that are selectively important for cancer stem cell self-renewal and survival. Unfortunately, though multiple pathways underlying tumorigenesis in cancer or self-renewal in embryonic and adult stem cells have been elucidated in the past, no pathways have been identified and validated for cancer stem cell self-renewal and survival.
There has also been a lot of research into the identification and isolation of cancer stem cells. Methods used mainly exploit the ability of CSCs to efflux drugs, or are based on the expression of surface markers associated with cancer stem cells.
For example, since CSCs are resistant to many chemotherapeutic agents, it is not surprising that CSCs almost ubiquitously overexpress drug efflux pumps such as ABCG2 (BCRP-1) [7-11], and other ATP binding cassette (ABC) superfamily members [12, 13]. Accordingly, the side population (SP) technique, originally used to enrich hematopoetic and leukemic stem cells, was also employed to identify and isolate CSCs [14]. This technique, first described by Goodell et al., takes advantage of differential ABC transporter-dependent efflux of fluorescent dyes such as Hoechst 33342 to define and isolate a cell population enriched in CSCs [10, 15]. Specifically, the SP is revealed by blocking drug efflux with verapamil, at which point the dyes can no longer be pumped out of the SP.
Researchers have also focused on finding specific markers that distinguish cancer stem cells from the bulk of the tumor. Most commonly expressed surface markers by the cancer stem cells include CD44, CD133, and CD166 [16-22]. Sorting tumor cells based primarily upon the differential expression of these surface marker(s) have accounted for the majority of the highly tumorigenic CSCs described to date. Therefore, these surface markers are well validated for identification and isolation of cancer stem cells from the cancer cell lines and from the bulk of tumor tissues.
Stat3 Pathway.
There are many different genetic defects in mammalian or human cancer cells, and many have been studied in the quest to cure cancer. For example, the p53 tumor suppressor has been found to be defective or altogether absent in more than half of the human cancers. The STAT (Signal Transducers and Activator of Transcription) protein family are latent transcription factors activated in response to cytokines/growth factors to promote proliferation, survival, and other biological processes. Among them, Stat3 is activated by phosphorylation of a critical tyrosine residue mediated by growth factor receptor tyrosine kinases, Janus kinases, or the Src family kinases, etc. These kinases include, but are not limited to EGFR, JAKs, Abl, KDR, c-Met, Src, and Her2 [23]. Upon tyrosine phosphorylation, Stat3 forms homo-dimers, translocates to the nucleus, binds to specific DNA-response elements in the promoter regions of the target genes, and induces gene expression [24].
In normal cells, Stat3 activation is transient and tightly regulated, lasting from 30 minutes to several hours. However, Stat3 is found to be aberrantly active in a wide variety of human cancers, including all the major carcinomas as well as some hematologic tumors. Stat3 plays multiple roles in cancer progression. As a potent transcription regulator, it targets genes involved in many important cellular functions, such as Bcl-xl, c-Myc, cyclin D1, Vegf, MMP-2, and survivin [25-30]. It is also a key negative regulator of tumor immune surveillance and immune cell recruitment [31-33].
Ablating Stat3 signaling by antisense, siRNA, a dominant-negative form of Stat3, and/or blockade of tyrosine kinases inhibits certain cancer cell lines or tumors in vitro and/or in vivo [24, 26, 34, 35]. But no clear link between Stat3 and cancer stem cell functionality has ever been empirically made. Nor have researchers found an effective Stat3 pathway inhibitor to explore potential therapeutic uses with regard to cancers that have been found to contain cancer stem cells. As described earlier, cancer stem cells (CSCs) have been recently demonstrated to be fundamentally responsible for tumorigenesis, metastasis, and reoccurrence, and should be taken into consideration in designing any curative therapy that targets a tumor known to have these cells no matter how small a fraction of the tumor mass they may constitute.
In diseases other than cancer, over-activation of Stat3 by Interleukin 6 (IL6) has been demonstrated in a number of autoimmune and inflammatory diseases [36]. Recently, it has been revealed that the Stat3 pathway also promotes pathologic immune responses through its essential role in generating TH17 T cell responses [37]. In addition, IL6-Stat3 pathway mediated inflammation has been found to be the common causative origin for atherosclerosis, peripheral vascular disease, coronary artery disease, hypertension, osteroprorosis, type 2 diabetes, and dementia.